Protective coatings for synthetic resin surfaces



Patented Me, was i v 2,487,254

Qumran S AT Es PAT ENT. OFFICE 2,431.84

PROTECTIVE COATINGS FOB SYNTHETIC nnsm summons Paul L. Mahoney, nelson Heights, N. in. am

to United States Rubber Company, Ncw.York, I N. Y., a corporation of New Jersey No Drawing. Application April I, 1948,

- Serial No. 19,639

2 Claims. (01. 111-0 This invention relatesto protective coatings for "i generally a a t condemfldn of plas ic glass and other rigid surfaces, andm l polyvinyl alcohol with butyraldehyde. Other Pa ularly to temporarily protecting such s polyvinyl acetal resins, such as polyvinyl formal. faces during handling, shipping. storage, and the polyvinyl t l, lyvin l heptal, -polyvlnyllike. j 5 propional,polyvinyl i'urfural which'are similarly Th pr s n inven ion r la s p m r ly to e produced by the condensation of polyvinyl alcohol. porary coatings for plastic glass and other rigid withthe appropriate aldehyde,,such as iorma; su aces fo o on du ha dlin shipp n dehyde, acetaldehyde, heptaldehyde. and so forth,

storage and the like that maybe readily peeled may also b used,

ofl in large pieces, and that on removal. as by In preparing the aqueous suspension of the stripping, will leave the plastic glass or other surpolyvinyl acetal resin, the resin, such as polyvinylface in an undamaged condition. The term butyral, may beplasticized as in an internal Plastic glass" is used in its commonly acc p e mixer. such as a Werner 8: Pileid'erer mixer, with sense as descriptive of the clear synthetic plastic castor oil in amounts of about to 100 parts resins which are available insheetandotherforms castor oil to 100' parts of polyvinyl acetal resin.

. and which may be shaped as by molding at rela- A hydrophllic colloidal dispersing agent, such as fi e y 1 temperatures. for am e. 3 F- to asoap, may bethen introduced into the plasticized 300F. Exampleso suc plas c lass are methyl resin, and watermay be slowly added until a m l'late resin, commercially available in change in phase takes place and the plasticized sheet form under the trade names Lucite" and 2 water-insoluble resin particles become dispersed Plexiglas, cellulose acetate, available under the in the aqueous medium. The method of disperstrade name Lumarith and other trade names, 1 ing various water-insoluble synthetic resins in an and ethyl cellulose. The present invention may 1 aqueous medium by introducing a hydrophilic also be used to protect plastic glass surfaces durcolloidal dispersing agent and subsequently addv ing shaping operations. I ing water while masticating the resin, as in an According to the present invention, asatisinternal mixer, is known. vSuch ,a method factory protective film or. coating may be sprayed eliminates the necessity for the use of volatile on a plastic glass or other rigid surface from an I organic solvent media as where resins are dis-'- aqueous suspension of a plasticized water-in solved in avolatilesolvent to-give a solvent solusoluble polyvinyl acetal resin if the solids contion or solvent dispersion, with or without subcentration of, the suspension is within the range sequent emulsifying in-water of such solvent of to 60% and preferably from to solution of the resin to give an aqueous emulsion The term "aqueous suspension? is used herein in of the organic solvent solution of the resin. Such its commonly accepted meaning as a colloidal solvent solutions or solventdlspersions of polydispersion of a solid in a liquid, viz., water; In vinyl acetal' resins and such aqueous emulsions such aqueous suspensions of polyvinyl acetals, the of these solvent solutions of the resin are unsolid polyvinyl acetal is the dispersed or dissatisfactory for spraying protective coatings'on continuous solid phase and the water is the explastic glass and other rigid surfaces for a variety ternal or continuous liquid phase. The preferred of reasons. Where the plastic glass is under polyvin l acetal resin is'polyvinyl butyral. The 40 stress, as from amanufacturing or shapingbperapolyvin l acetal resin must be plasticized before tion, or where it is in a frame and subject to it is dispersed in the water and also in order to expansion and contraction, organic solvents will form a readily stripable protective film on the very readily surface craze the plastic glass. The

. article to be protected. The plasticizer 'must be organic solvent must be recovered, which is diflicompatible with the polyvinyl acetal so that it 5 cult, if not impossible, with large articles where will not bleed out and migrate. to the surface it is impractical to use closed tunnels. or ,the like, of the article being protected, and it must be. with adequate recovery systems in the case of non-reactive or incompatible with the article to hand manipulated spraying operations. The be protected so that it will notinjure the surface 7 organic solvent vapors are toxic to operators 01' of the article. The preferred plasticizer for-the 50 the spray gun equipment, and there is the added polyvinyl acetal is castor oil, which is compatible fire and-explosion risk with many organic solvents.

with polyvinyl acetals and incompatible with" With the aqueous suspensions of the plasticized polymerized methyl methacrylate plastic glass. polyvinyl acetal of the present invention, large The castor oil may be raw or blown. Polyvinyl sections of plastic'glass, and other large surfaces. acetal resins are well known. Polyvinyl butyral asforexample, an entire airplane, maybe sprayed in the open at room or elevated temperatures. without danger to the operatives, to give a satisof the plastic glass.

factory readily peelable protective coating. The solids concentrations of the aqueous suspensions of the plasticized polyvinyl acetal must, however,

be kept within a definite range in order to give satisfactory spraying. The total solids concentration of the aqueous suspension of the polyvinyl acetal resin should not be above 6096, otherwise,

, otherwise the physical properties of the protective film would be adversely affected by water, as from rain or moist air. For example, a protective film of a; water-soluble partial polyvinyl acetal (which contains a large proportion of hydroxyl and/or .011 under these conditions.

4 resin powder in a Werner I: Pileiderer mixer hea'tedto around 100". C., was added 50 parts of castor oil and the mixing continued until a smooth plasticized mass was obtained. The mixer was allowed to cool to approximately room temperature and 5 parts of a fatty acid, oleic acid, were mixed well into the plasticized resin, followed by a small amount of water. Suillcient caustic alkali dissolved a small amount of water was added to the mass to neutralize the oleic acid. The alkali will not saponify the castor Water was added slowly with constant pulling of the plastic mass by'the rotating blades of the mixer until an inversion of phase took place, and further water was added to the desired solids content of about 50% solids. Such a dispersion ma readily be applied to surfaces to be protected as by dipping or spraying. Preferably when used as a spray a small amount of thickener, such as casein is added to the mass in the mixer with the addition of sufficient further alkali or of borax to 'solubilize the casein before inversion of phase acetyl groups) if exposed to rain or water, as on 1 an airplane or on a plastic glass section thereof on the deck of a ship, would swell and become gelatinous and lose its strength so that it could not be peeled off in large sheets.

In order to utilize polyvinyl acetal resins as temporary protective coatings to be stripped from the article after there is nofurther need for such protection, the resin which is itself too hard in film form for such use must be severely plasticized. This plasticization may be made to take place according to the present invention with castor oil where the polyvinyl acetal resin is applied to methyl methacrylate resin, since eastor oil has no detrimental affect on a plastic glass such as methyl methacrylate resin. The plasticization, of the polyvinyl acetal with the conventional type of ester plasticizers, such as dibutyl phthalate, tricresyl phosphate, and the like,

- causes surface crazing of the polymerized methyl methacrylate plastic glass on removal of the protective covering by virtue of the reaction of such ester plasticizers on the plastic glass. Such surface crazing of the plastic glass may entirely ruin the plastic glass, or if not entirely ruined, would necessitate the arduous and time-consuming operations of a final polishing of the surface Where the polyvinyl acetal is to be applied to articles which are resistant to the effect of such ester plasticizers, as in the protection of wood and metal surfaces, then the polyvinyl acetal may be plasticized with such conventional ester plasticizers before dispersing in water. The castor oil used as a plasticizer for the polyvinyl butyral will permit a film or coating of such plasticized resin to be applied from an aqueous suspension of the same onto a plastic glass surface and to remain incontact with such surface at room temperatures, or at elevated temperatures such as those used for shaping the plastic glass, as for example. 135 F. to 300 F. without the detrimental eifect of surface crazing caused by the usual ester type plasticizers commonly used for synthetic resin materials.

The following illustrates the preparation of an aqueous suspension of polyvinyl butyral for use as a protective covering according to the pres ent invention. 7

To 100 parts of commercial polyvinyl butyral is made to take place. Such a dispersion, preferably at around 40 to 50% solids, may readily be applied to the plastic glass or other rigid surface in the form of a coating or film of the thickness adequate for the requisite protection The aqueous suspensions of polyvinyl acetal resin used in the process of the present invention are thus preferably substantially free from volatile organic solvents, although volatile solvents in amount up to 10% by weight of the water content of the suspension of polyvinyl acetal resin may be added without detrimental effect to the protective film except to somewhat decrease the tensile strength of the film. There does not appear any reason for such volatile solvent additions, since the resin will dry in air readily without the use of volatile solvents. The aqueous suspension of the polyvinylacetal resin should, of course, be free from any volatile solvents, as well as any plasticizers, of the types which will attack the material of the surface of the article to be protected. Where plastic glass sheeting is to be shaped, as by molding, either with a pressure or a vacuum mold, a film of from .002 to .005 inch is generally sufficient for protection of the face or faces of the plastic glass sheeting which come in contact with forming surfaces of the mold. i

The aqueous suspension of polyvinyl acetal resin containing the castor oil may be applied to the plastic glass or other surface to be protected and the film dried at room temperature, or if desired, at somewhat elevated temperatures to increase the drying rate. The aqueous suspension of the polyvinyl acetal resin may be applied to the plastic glass surface of completely the plasticized polyvinyl acetal to one or both surfaces of the plastic glass sheeting as by dipping or spraying, and the molding or other oper- This application is a continuation-in-part of ations may'then be performed with the protec--v I tive coating of the polyvinyl acetal resin adhered to the plastic glass throughout such operations. After the shaping or other operations, the protective film of polyvinyl acetal resin may be permitted to remain on the plastic glass through assembly and during further handling, shipping, and storage until it is desired to remove the protective film for the destined use of the article. The polyvinyl acetal film with the castor oil plasticizer may be readily stripped from the plastic glass surface and no etching or surface crazing of the plastic glass results. While the use of an aqueous suspension of a water-insoluble polyvinyl acetal resin containing the castor oil has been described herein specifically with respect to the protection of plastic glass surfaces, it may also be used for the protection of rigid surfaces generally, such as wood, and polished and enamelled metal, as for example by application to an entire airplane, where it is desired to protect all the surfaces during handling, shipping, storage, and the like, and to provide a coating which may be readily peeled off in large pieces leaving the protected surfaces in unharmed condition, and with no need for expending any time in bringing the protected surfaces to pristine condition when there is no further need for the protection. The protective coating of the polyvinyl acetal resin containing the castor oil may be applied to plastic glass sheeting from an aqueous suspension at the place of manufacture instead of the usual paper covering adhered to the plastic glass by means of a so-called pressure-sensitive adhesive. Coatings of the polyvinyl acetal resin containing castor oil may beredispersed in water after stripping, with or without the addition of further castor oil, in the same manner as illustrated above.

"1943, and now abandoned.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. The method of protecting a methyl'methacrylate surface during handling, shipping, storage and the like which comprises applying to said surface a protective coating which may be readily peeled off in large pieces by directly depositing on said surface the solids of an aqueous suspension of a castor oil plasticized water insoluble polyvinyl acetal resin having a solids concentration within the range of 35 to 60%, said aqueous suspension of polyvinyl acetal resin containing not more than 10% by weight of the water content of the suspension of volatile organic solvent.

2. The method of protecting a methyl methacrylate surface during handling, shipping, storage and the like which comprises applying to said surface a protective coating which maybe readily peeled off in large pieces by directly depositing on said surface the solids of an aqueous suspension of a castor oil plasticized water-insoluble polyvinyl butyral resin having a solids concentration within the range of 40 to said aqueous suspension of polyvinylbutyral resin being substantially free of volatile organic solvent.

PAUL L. MAHONEY.

REFERENCES CITED The following references are of record in the Richards et a1. Aug. 3, 1945 

